Abandoned Sawmill - Skellige
Location: Abandoned Sawmill
Quest Giver: Acquired on approaching the area
Other Requirements: N/A
This quest begins when you approach the Abandoned Sawmill area on Skellige. You'll find this location to the southwest of Gedyneith, the area where the Coronation Quest <Add Link> takes place. To accept this quest all you must do is approach the area - it'll automatically be added to your log when you do.
If you're having trouble locating the Abandoned Sawmill use my screen shot below for additional guidance. Technically there are four locations that this quest has you examine but you only need to find one in order to complete/advance the quest (shown in the screen shot below).
To spawn the Leshen which you'll have to defeat to complete this quest, investigate the human bones that are outside of the small building along the northern portion of town (pictured above). They are difficult to see so make sure to use your Witcher Senses to find them.
As for the fight with the Leshen, he's just like any other Leshen - except much easier since he isn't a named of any sort. Defeating him completes the quest.
Leshen Weaknesses: Dimertium Bomb, Relict Oil, Igni